1. Field of the Invention
The present invention relates to an electroluminescent (EL) lighting element preferably used for illumination of various electronic components, and a manufacturing method of the EL lighting element. Furthermore, the present invention relates to an illuminated switch unit using this EL lighting element preferably applicable to input operating sections of various electronic devices.
2. Prior Art
Recent developments of micro IC drive inverters have increased the need for an EL lighting element which is thin and capable of realizing a proper surface lighting serving as a backlight for liquid crystal display units and switches of various electronic components, such as communication devices, video devices, and acoustic devices.
FIG. 6 is a plan view showing this kind of conventional diffusion-type EL lighting element. FIG. 7 is a cross-sectional view taken along a line A-B of FIG. 6. A transparent conductive film 112 is formed on an upper surface of an insulating transparent film 101, such as polyethylene terephthalate (PET) film, by depositing stannic indium oxide on the entire surface of the insulating transparent film 101 by sputtering. A light-emitting layer 103 is pattern printed on the transparent conductive film 112 by screen printing or the like, and is then dried. Composition of light-emitting layer 103 itself is made by dissolving high-dielectric resin, such as cyanoethyl pullulan or vinylidene fluoride group rubber, in organic solvent, such as dimethyl formamide or N-methyl pidoridone, and then diffusing illuminant, such as zinc sulfide, therein. A dielectric layer 104 is formed on light-emitting layer 103. Dielectric layer 104 contains high-dielectric material, such as barium titanate, diffused in a similar resin used in light-emitting layer 103.
Furthermore, a back-surface electrode layer 105 is formed on dielectric layer 104 by using a carbon resin group paste or a silver resin group paste. An insulating coat layer 106 is formed on back-surface electrode layer 105 by using an insulating paste. These dielectric layer 104, back-surface electrode layer 105 and insulating coat layer 106 are laminated in this order by pattern printing and drying each of them successively. A collecting electrode 105b of transparent conductive film 112 is made by partly extending transparent conductive film 112 from the boundary of the laminated layers of light-emitting layer 103, dielectric layer 104, back-surface electrode layer 105 and insulating coat layer 106, and pattern printing a silver resin group paste etc. on thus exposed surface of transparent conductive film 112 and then drying the printed paste. Meanwhile, a collecting electrode 105a of back-surface electrode layer 105 is made by exposing an end region of back-surface electrode layer 105 from insulating coat layer 106.
Furthermore, in a case where a distance between collecting electrodes 105a and 105b needs to be elongated, it is possible to add an insulating film 113, such as epoxy group resin, to compensate for the poor bondability between the high-dielectric resin used in dielectric layer 104 and transparent conductive film 112, as shown in FIG. 8.
However, according to the above-described conventional arrangement, PET film on which transparent conductive film 112 is formed by sputtering is expensive. If collecting electrodes 105a and 105b of the EL lighting element are extended externally, the PET film with transparent conductive film 112 will be necessarily elongated extensively out of the light-emitting region, resulting in an increase of the cost. Furthermore, using anisotropic conductive adhesive for connecting the electrodes of such an EL lighting element to the printed circuit board is disadvantageous in that an additional insulating layer needs to be specially provided on the PET film with transparent conductive film 112 entirely formed by sputtering and, as a result, an overall thickness of the EL lighting element will be so increased that the heat conductivity is worsened and the resultant bonding is not satisfactory. Yet further, there is a possibility that a short-circuit may occur in a connection using a connector because transparent conductive film 112 is located along the entire periphery of the PET film. Hence, the connector connection will be not practically adopted. Still further, a method of removing transparent conductive film 112 in advance by etching in the region other than the light-emitting region is further expensive.
In view of the foregoing, the above-described problems can be solved by pattern printing the transparent electrode by using conductive paste having light permeability. This kind of light permeable conductive paste is already known as disclosed in Unexamined Japanese Patent Application Nos. 64-10595 and 63-10496. However, printing the light-emitting layer and the dielectric layer in a piled-up manner on the coating surface of this light permeable conductive paste will cause the following problem. Due to the presence of polar solvent, such as dimethyl formamide or N-methyl pidoridone, used for dissolving the high-dielectric resin to be involved in the light-emitting and dielectric layers, the conductivity of the EL lighting element is fairly deteriorated when high temperature is applied in the drying procedure which is mandatorily required for this kind of solvent. It means that there is a possibility that no light is emitted or very poor light emission is obtained only in the limited region near the collecting electrodes. Hence, it is not possible to print the light-emitting layer and the dielectric layer in the piled-up manner on the coating surface of this light permeable conductive paste, although it is possible to use this for back-surface electrode layer 105 or to form the transparent electrode layer and light-emitting layer 103 independently and later connect them by the laminate or the like.
Hereinafter, a conventional illuminated switch unit using such an electroluminescent lighting element will be explained.
As shown in FIG. 14, an upper insulating sheet 201 made of resin film has a reverse surface on which a movable contact 202 is printed. A lower insulating sheet 203 has an upper surface on which a stationary contact 204 is printed. Spacer 205, which is resin film having both surfaces applied with adhesive, are interposed between upper insulating sheet 201 and lower insulating sheet 203 so that movable contact 202 and stationary contact 204 are disposed in a confronting relationship with a predetermined gap therebetween when these sheets 201 and 203 are fixed to each other to form a membrane switch 206.
An electroluminescent lighting element (abbreviated as EL lighting element) 207 is disposed on this membrane switch 206. A top sheet 211, made of transparent resin film, is disposed and fixed on this EL lighting element 207 via spacer 212 of transparent resin film having adhesive applied on both surfaces thereof. Top sheet 211 has an upper surface provided with printed pattern representing letters, figures or others 208 and a lower surface provided with a button 209 which is a transparent or semi-transparent resin product bonded by adhesive 210 to the lower surface of top sheet 211, thereby constituting the conventional illuminated switch unit.
Next, an operation of the above-described conventional illuminated switch unit will be explained. Depressing top sheet 211 causes button 209 to move downward and press or push EL lighting element 207. Hence, a region of upper insulating sheet 201 which received a depressing force through EL lighting element 207 is recessed. Movable contact 202, which constitutes part of membrane switch 206 and is provided at the lower surface of the recessed portion of upper insulating sheet 201, is depressed downward. Hence, movable contact 202 can be brought into contact with stationary contact 204 provided on lower insulating sheet 203, thereby establishing an electrical connection therebetween.
However, according to the above-described conventional arrangement, the presence of button 209 and spacer 212 causes undesirable illumination such as a silhouette of these parts or irregularities of luminance. Furthermore, the manufacturing costs will be increased due to numerous parts and increased assembling processes.